What causes the temperature to decrease with height in the troposphere?

We have all noticed that if we hike from a place with low elevation and go up at a place with high elevation, that the temperature is gradually dropping, thus giving us a clue that the lower places are warmer, while the higher places are cooler. The reason for this, even though both the places with low and high elevation are located in the troposphere, thus the same atmospheric layer, is that the density of it is not the same everywhere. The troposphere has the highest density at its lowest points, and as the height increase the density decreases. Since the higher parts are less dense, that means that there are less gases, thus there are less molecules that are further apart from each other. This results in lesser absorption of heat, so this parts are cooler, while the lower parts are warmer because the troposphere is denser, there's more molecules that are also closer to each other, thus they absorb more heat.

the first plants to make the transition

explanation: